1. Field of the Invention
The present invention relates to a quantum effect device which utilizes a quantum effect such as tunnel effect.
2. Description of the Related Art
In recent years, it has been proposed that a logic circuit be assembled without using three-terminal transistors such as bipolar transistors. (See Craig S. Lent et al., Quantum Cellular Automata Nanotechnology, Vol. 4, pp. 49-57.)
FIGS. 18A and 18B schematically show an inverter circuit incorporating no conventional transistors, which is called a "cell-connected inverter circuit." As FIGS. 18A and 18B show, the cell-connected inverter circuit comprises six 5-quantum dot cells C1 to C6. A 5-quantum dot cell has five quantum dots D1 to D5, two of which contain one electron each, as illustrated in FIGS. 19A and 19B. It can assume two recognizable, stable ground states .PSI..sub.1 and .PSI..sub.2 since the two electrons undergo Coulomb interaction and repel each other, trying to acquire a lower energy level.
How the cell-connected inverter circuit operates will be described, with reference to FIGS. 18A and 18B. An input "1" is supplied to the 5-quantum dot cell C1, setting the cell C1 into ground state .PSI..sub.1. Then, the 5-quantum dot cell C2 takes ground state .PSI..sub.1 which is electrically stable with respect to the cell C1. Further, the 5-quantum dot cell C3 assumes ground state .PSI..sub.1 which is electrically stable with respect to the cell C2. Next, the 5-quantum dot cell C4 assumes ground state .PSI..sub.2 which is electrically stable with respect to the cell C3, and the 5-quantum dot cell C5 assumes ground state .PSI..sub.2 which is electrically stable with respect to the cell C4. Finally, the 5-quantum dot cell C6 takes ground state .PSI..sub.2 which is electrically stable with respect to the cell C5. As a result, the cells C1 and C6, which are the input and output cells, respectively, assume different ground states. The cell-connected inverter circuit therefore outputs "0" when it receives "1."
The cell-connected inverter circuit is, however, disadvantageous in one respect. When the input binary value is changed from "1" to "0, " the 5-quantum dot cells C1 to C3 assume ground state .PSI..sub.2, but the 5-quantum dot cells C4 to C6 remain in ground state .PSI..sub.2, as is illustrated in FIG. 18B. In this case, the output binary value is identical to the input binary value. In other words, the circuit fails to function as an inverter.
A plurality of 5-quantum dot cells can transfer a signal if they are arranged in a row or a column. When a bias is applied on the first 5-quantum dot cell, thereby setting this cell into, for example, ground state .PSI..sub.1 the second 5-quantum dot cell assumes the same ground state (.PSI..sub.1) due to its Coulomb interaction with the first cell. Similarly, the third to sixth 5-quantum dot cells assume ground state .PSI..sub.1. Therefore, the ground state .PSI..sub.1, is transferred as a signal from the first cell to the last via the remaining cells. When the first 5-quantum dot cell is set into ground state .PSI..sub.2, this ground state is transferred as a signal from the first cell to the last via the remaining cells.
If 5-quantum dot cells are arranged in a row or a column, they can transfer a signal (either ground state) along a straight path only. In an integrated circuit, 5-quantum dot cells must be arranged, forming bent paths and branched paths. Otherwise they could not change the direction of transferring signals or branch a signal transfer path in order to supply the signals to an arithmetic logic unit or a memory located at a given position in the integrated circuit.
To change the direction of transferring signals, 5-quantum dot cells may be arranged as shown in FIG. 48. To branch a signal transfer path, they may be arranged as shown in FIG. 49. Both arrangements are those proposed by Lent et al.
The cell arrangement shown in FIG. 48 is a so-called "bent-back wire" consisting of five 5-quantum dot cells C1 to C5. A signal S is first transferred through the cells C1 to C3 in the direction of 3 o'clock and then through the cells C3 to C5 in the direction of 6 o'clock. The bent-back wire has a problem, however. The 5-quantum dot cells C2 and C4 are unstable. They do not always assume a ground state since one of the electrons in the cell C2 and one of the electrons in the cell C4 are so close to have Coulomb interaction and repel each other, inevitably rendering the states of the cells C2 and C4 electrically unstable. It is difficult for the bent-back wire to change the direction of transferring the signal S, while maintaining the information which the signal S conveys.
The cell arrangement shown in FIG. 49 is a so-called "multi fan-out branched wire" consisting of ten 5-quantum dot cells C1 to C10. In the multi fan-out branched wire, the input signal S is branched twice--first at the cell C3, and then at the cell C5. As a result, the signal S is split into two signals S.sub.1 and S.sub.2 which are transferred in the direction of 3 o'clock along two parallel paths. The multi fan-out branched wire has a problem, too. Since the cells C4, C6, C7 and C9 are not electrically stable, it is difficult for the multi fan-out branched wire to split the input signal S into two signals S.sub.1 and S.sub.2, while maintaining the information which the signal S conveys.
C. S. Lent et al. have proposed that a plurality of 5-quantum dot cells be combined to constitute a quantum effect device named "cellular automaton."(See Appl. Phys. Lett. 62 (1993), p. 714. ) As diagrammatically shown in FIG. 42, a 5-quantum dot cell is rectangular and comprised of five spherical quantum boxes. Each quantum box is small enough to confine electrons in 0-dimensional fashion. Of these five quantum boxes, four are located in the corners of the cell, and the remaining one is located at the center of the cell. Two of the five quantum boxes, i.e., boxes 1017, contain one electron each.
The five quantum boxes are arranged so close to one another that electrons can move back and forth between any two adjacent quantum boxes by virtue of tunnel effect. In contrast, the 5-quantum dot cells constituting a quantum effect device are spaced apart by so long a distance that electrons cannot move among the cells despite tunnel effect.
The two electrons in one 5-quantum dot cell repel each other due to Coulomb interaction, and assume the lowest energy level when they are at the ends of a diagonal. Therefore, the 5-quantum dot cell can take two electrically stable states ("0" and "1"). Namely, the cell is bistable.
Since 5-quantum dot cells are bistable (0,1), they can form a memory when arranged close to one another, each for storing one bit of information. For example, four 5-quantum dot cells may be arranged in a row as shown in FIG. 43. If this is the case, when state "0" is input to the left cell, it will be transferred to the right cell via the intermediate two cells--by virtue of the Coulomb repulsion acting between the two electrons contained in each 5-quantum dot cell.
Obviously, 5-quantum dot cells can form a wire for transferring electric signals only if they are arranged close to one another. Moreover, 5-quantum dot cells can constitute a logic circuit if they are arranged in an appropriate pattern.
However, 5-quantum dot cells of the type described above have the following problem.
As indicated above, the conventional 5-quantum dot cell is rectangular, and the five quantum boxes in the cell are spherical. It follows that any two adjacent quantum boxes arranged in a diagonal are less spaced apart than are the two adjacent quantum boxes arranged in a line parallel to any side of the cell. The tunneling probability is therefore high, and electrons can move between any adjacent quantum boxes due to tunnel effect.
Certainly, the cell indeed remains in a specific state while a bias is being applied to it. When the application of the bias to it is stopped, however, its state becomes less stable with time. Therefore, the conventional 5-quantum dot cell cannot be used in, for example, a nonvolatile memory.
A plurality of the conventional 5-quantum dot cells may be arranged to form an optical memory. Then, light (optical bias) is applied to each cell to write data into it. The cell stays in a specific state while being irradiated with light, but its state becomes unstable after the application of light has been terminated. To enable the optical memory to perform its function well, the cells must be connected by wires. There is then no longer an advantage in using 5-quantum dot cells, which might not otherwise be necessarily connected by wires.
The conventional 5-quantum dot cells may be arranged, forming a logic circuit. In this case, a problem arises. As described above, electrons move sideways as well due to tunnel effect. Consequently, each cell has its threshold energy decreased too much, and operates erroneously.
The conventional quantum effect device is disadvantageous in the following respect.
A quantum effect device of QIC (Quantum Interconnections with Cellular Automata) type can be operated only if its constituent quantum cells remain in ground state. The operating temperature of the device depends on the energy difference between the ground state and the primary excited state. The energy difference is determined by the Coulomb interaction among the quantum cells (e.g., 5-quantum dot cells). More specifically, the more each quantum cell is influenced by the adjacent quantum cells, the greater the energy difference. Hence, the shorter the distance among the quantum cells, the higher the operating temperature of the quantum effect device. As the distance among the quantum cells is reduced, however, the Coulomb interaction among them becomes prominent, impairing the bistable state of the quantum cells. Inevitably, the quantum effect device fails to perform its function.